Advances in technology and increasing demand associated with mobile equipment have led to a sharp increase in demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and operating voltage, long lifespan and low self-discharge are commercially available and widely used.
In addition, in recent years, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles (EVs) and hybrid electric vehicles (HEVs) as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are major causes of air pollution. Nickel metal hydride (Ni-MH) secondary batteries are generally used as power sources of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. However, research associated with use of lithium secondary batteries having high energy density, high discharge voltage and power stability is actively underway and some of such lithium secondary batteries are commercially available.
A lithium secondary battery has a structure in which a non-aqueous electrolyte comprising a lithium salt is impregnated into an electrode assembly comprising a cathode and an anode, each comprising an active material coated on a current collector, and a porous separator interposed therebetween.
Lithium transition metal oxides such as lithium cobalt-based oxides or lithium nickel-based oxides are commonly used for a cathode for lithium secondary batteries. Use of lithium metal oxide having a spinel structure such as LiNixMn2−xO4 (in which x is 0.01 to 0.6) which has a mean voltage of 4.7V and is used in high-voltage batteries is considered.
However, the lithium metal oxide having a spinel structure has a high voltage. In this case, the lithium metal oxide reaches an oxidation potential of an electrolyte, thus leading to oxidation of the electrolyte and thus generation of by-products and deteriorating secondary batteries.
Accordingly, there is an increasing need for methods capable of solving these problems.